Moving coil armature electrical instrument



May 23, 1950 D. A. YOUNG ETAL MOVING COIL ARMATURE ELECTRICAL INSTRUMENT 3 Sheets-Sheet 1 Filed Dec. 2'7, 1944 WITNESSES:

INVENTOR5 Do BzYwz /vcejlazms.

. ATTORNEY May 23, 1950 D. A. YOUNG ET AL 2,508,439

MOVING COIL ARMATURE ELECTRICAL INSTRUMENT Filed Dec. 27, 1944 AZM ATTORN EY May 23, 1950 o. A. YOUNG EI'AL MOVING CQILARMATURE ELECTRICAL INSTRUMENT M V s w w a m? m m V m S E O a; WM w m if A w Z? a 3 j 8 illl E .|,\-|.7a NHL 2/ fi pmz wy v a j a 4 A w m a w c m w W. l 1 F Patented May 23, 1950 MOVING con. Amu'rmm ELECTRICAL msrammnr Douglass A. Young, East Orange, and Lawrence J.

Lunar, Cedar Grove, N. 1., assignors to Westinghouse Electric Corporation, East Pittsburgh, PL, a corporation of Pennsylvania Application December 27, 1944, Serial No. 570,029

21 Claims.

I This invention relates to permanent-magnet, moving-coil electrical instruments, and it has ;,*particular relation to permanent-magnet, moving-coil instruments wherein a moving coil is mounted for rotation relative to a magnetic structure about an axis intermediate two sides of the coil.

In certain prior-art permanent-magnet, moving-coil electrical instruments, 8. moving coil is mohnted for rotation with respect to a magnetic structure about an axis intermediate two sides of the coil. The magnetic structure is configured to provide air gaps within which the two sides of the moving coil are positioned for movement. In addition, the magnetic structure includes permanent-magnet means for producing magnetic fields in the air gaps. The magnetic structure includes a magnetic core positioned within the moving coil. This magnetic core establishes a low reluctance magnetic path for magnetic flux supplied to the air gaps. Such instruments are well known in the art and are employed, for example, as electrical relays and as electrical measuring instruments for indicating or recording the values of various quantities.

To remove a moving coil from its operative position relative to its associated magnetic structure in an electrical instrument of the type described in the preceding paragraph, it is necessary to remove the magnetic core from the remainder of the magnetic structure. The removal of the magnetic core is objectionable for several reasons. It is difficult to separate and reassemble parts of a magnetic structure without changing the magnetic characteristics thereof in some degree. Furthermore, the removal of the magnetic core introduces an extremely large air gap in the magnetic circuit associated with the permanent magnet means which supplies magnetic flux to the air gaps. The presence of a large air gap in this magnetic circuit may result in demagnetization of the permanent magnet means. To permit such demagnetization, it is customary to replace the magnetic core by a temporary magnetic keeper as the magnetic core is removed. This practice requires an additional part in the form of a magnetic keeper and also complicates the disassembly and assembly of the instrument. It a magnetic core must be removed and reassembled with the moving coil assembly, the possibility of damage to the moving coil and its pivots during such assembly or disassembly is greatly increased. In addition, during assembly it is necessary to align not only the moving coil but also the removable magnetic core with the remainder of the magnetic structure.

In accordance with the invention, a permanentmagnet, moving-coil electrical instrument is provided wherein a moving coil is mounted for rotation relative to a magnetic structure about an axis intermediate two sides of the coil. The magnetic structure provides air gaps within which the two sides of the moving coil are positioned for rotation. In addition, the magnetic structure is configured to provide a passage through which the moving coil may be removed from its operative position with respect to the magnetic structure without disturbing the magnetic structure in any way. Consequently, an instrument designed in accordance with the invention avoids the objections set forth in the preceding paragraph. The invention contemplates further the provision of a bracket within which the moving coil is positioned for rotation and which may be removed with the moving coil from its associated magnetic structure without disturbing the magnetic structure.

Briefly, the magnetic core of a prior-art permanent-magnet, moving-coil instrument is replaced by a pair of magnetic cores or inner pole pieces which coact to form a resultant magnetic core. These pole pieces are spaced apart to provide a passage through which a movin coil may be inserted from a position external to its associated magnetic structure to a position wherein the moving coil may be rotated through a predetermined path to embrace its resultant magnetic core. A reverse procedure may be employed for removing the moving coil from its operative position relative to the magnetic structure to a position external to the magnetic structure. The magnetic structure preferably is so configured that substantially the only gaps in the magnetic circuits provided thereby for magnetic flux are the air gaps in which the two efl'ective sides of the moving coil are positioned for movement. If it is desired to connect the two inner pole pieces, the connecting element should be so located that when the moving coil is positioned for removal through the aforesaid passage, the connecting element is external to the coil and displaced from the aforesaid predet'ermined path.

In a preferred embodiment of the invention, each of the air gaps within which one of the effective sides of the moving coil is positioned for movement is provided with an independent magnetic circuit for establishing a magnetic field within the air gap. These independent magnetic circuits are spaced to define the aforesaid passage through which the moving coil may be moved from a position external to the magnetic structure to a position wherein the moving cell may be rotated through the aforesaid predetermined path to embrace its resultant magnetic core. In the preferred embodiment, the magnetic structure provides a soft magnetic body substantially surrounding the associated permanent magnet means and the moving coil to provide an effective magnetic shield therefor.

It is, therefore, an object of the invention to provide an improved permanent-magnet, movingcoil electrical instrument wherein th moving coil is mounted for rotation with respect to an associated magnetic structure about an axis intermediate two sides of the moving coil.

It is a further object of the invention to provide a permanent-magnet,- moving-coil electrical instrument wherein a coil is mounted for rotation relative to an associated magnetic structure about an axis intermediate two sides of the coil and wherein the magnetic structure is configured to permit removal of the coil from operative position with respect to the magnetic structure without disturbing the magnetic structure.

It is a still further object of the invention to provide a permanent-magnet, moving-coil instrument wherein a moving coil is mounted in a bracket for rotation relative to an associated magnetic structure and the bracket about an axis intermediate two sides of the coil and wherein the magnetic structure is configured to permit removal of the coil and bracket as a unit from operative position with respect to the magnetic structure.

It is also an object of the invention to provide a permanent-magnet, moving-coil instrument wherein a moving coil is mounted for rotation with respect to a magnet structure and wherein the magnetic structure provides a pair of magnetic paths, each of which establishes a magnetic field for a separate side of the coil, and wherein the magnetic paths are spaced to define a passage through which the coil may be removed from operative position relative to the magnetic structure.

It is an additional object of the invention to provide a permanent-magnet, moving-coil instrument which is substantially insensitive to magnetic fields produced externally of the instrument.

Other objects of the invention will be apparent from the following description, taken in conjunction with the accompanying drawings, in which:

Figure 1 is a schematic view of a prior-art permanent-magnet, moving-coil electrical instrument;

Figs. 2 to 9, inclusive, are schematic views of embodiments of permanent-magnet, moving-coil electrical instruments which illustrate the invention;

Figs. to 15, inclusive, are schematic views showing connections of moving cofl assemblies suitable for certain of the permanent-magnet, moving-coil electrical instruments illustrated in Figs. 2 to 9, inclusive; Figs. 16 to 21, inclusive, are views in perspectiye, with parts broken away, of various permane'nt-magnet, moving-coil electrical instruments embodying the invention;

Fig. 22 is a view in perspective, with parts broken away, of a bracket assembly, together with a pole piece and permanent magnet assembly suitable for the instrument shown in Figs. 20 and 21;

Fig. 23 is a view in elevation, with parts broken away, of a bracket assembly and a moving coil 4 assembly suitable for an electrical instrument embodying the invention;

Fig. 24 is a view in bottom plan. with parts broken away, of a modification of an electrical instrument embodying the invention.

Referring to the drawings. Fig. 1 shows a prior art permanent-magnet, moving-coil electrical instrument which includes a cylindrical magnetic core I and a pair of outer magnetic pole pieces 2 and 3 which are spaced from the magnetic core I to define therewith a pair of air gaps I and I. The magnetic core and the pole pieces form parts of a magnetic structure which is completed by s magnetic conductor 9. The magnetic core, the pole pieces and the magnetic conductor may be formed, all or in part, of permanent magnet material which is magnetized to produce a flow of magnetic flux in the direction indicated by the arrow If part of the magnetic structure is formed of permanentmagtiet material, the remainder may be formed of a soft magnetic material, such as soft steel. With the direction of flow of magnetic flux illustrated in Fig. 1, the pole piece 3 may be termed a north pole indicated by the reference character N, whereas the pole piece 2 is a south pole B.

A moving coil I I surrounds the cylindrical magnetic core I and is secured to a shaft I3 for rotation about the axis of the shaft. This moving coil has one side II positioned for movement through the air gap I and a second side II positioned for movement through the air gap I. It will be observed that the shaft I I is positioned intermediate the two sides II and I1. As well understood in the art, when an electrical current is directed through the coil I I, the current flowing through the coil side II coact-s with the magnetic fleld in the air gap I to produce a torque acting on the coil II about the shaft II. In addition, current flowing through the coil side II produces an additional torque acting on the coil about the shaft I3. Movement of the coil in response to the resultant torque acting thereon is opposed by a spring (not shown in Fig. 1).

As previously pointed out. the removal of the moving coil II from the instrument illustrated in Fig. 1 requires the removal of the magnetic core I from the remainder of the magnetic structure. The removal of the magnetic core I is ob- Jectionable for the reasons that it requires a magnetic keeper for the remainder of the magnetic structure, it tendsto change the resultant characteristics of the magnetic structure upon reassembly thereof, it increases the possibility of damage to the moving coil sue the pivots associated therewith and it necessitates the realignment of the magnetic core with the remainder of the instrument after each reassembl thereof.

To facilitate assembly and disassembly of the instrument, the magnetic core I is replaced by two magnetic parts I! and II (Fig. 2) which are spaced apart to provide a passage through which a moving coil may be moved. The magnetic parts is and 2| may be termed magnetic cores which coact to form a resultant magnetic core or they may be termed inner magnetic pole pieces. The magnetic structure of Fig. 2 otherwise is similar to that of Fig. 1.

In Fig. 2, a coil assembly 23 is provided which corresponds to the coil II of Fig. 1. This moving coil assembly is mounted on a shaft 25 for rotation about the axis of the shaft. If desired, the coil assembly 23 may be similar to the moving coil II of Fig. 1. However, since the inner pole pieces I! and 2| are separated from each other, the coil assembly 23 may be formed oi two moving coils each embracing a separate one of the inner pole pieces I! and 2|. Such a coil assembly will be illustrated and discussed below. It should be noted that the shaft 25 may be a through shaft extending completely through the space between the inner pole pieces l9 and 2|.

The space between the inner pole pieces i3 and 2| provides a passage 21 which is proportioned to permit movement of the coil assembly 23 therethrough from a position external to the magnetic structure to a position wherein the coil may be rotated to advance its sides 23 and 3| into the associated air gaps 5 and 1. This greatly facilitates the assembly and disassembly of the instrument. For example, let it be assumed that the coil assembly 23 is to be removed from its associated magnetic structure. To effect such removal, the coil assembly 23 may be rotated in a counterclockwise direction, as viewed in Fig. 2, from the position illustrated in full lines through a predetermined path to the position illustrated in broken lines. When the coil is in the position illustrated in broken lines, it may be moved in a direction parallel to the shaft 25 or transverse thereto to a position external to the magnetic structure. It should be noted that such removal of the coil does not disturb the magnetic circuit in any way. Consequently, the objections noted with respect to Fig. 1 are completely avoided. An opposite procedure may be followed to reinsert the coil assembly 23 in its operative position with respect to its associated magnetic structure.

The separation of the inner pole pieces I! and 2| introduces a very appreciable air gap represented by the passage 21 into the magnetic circuit through which the magnetic flux flows. Although the instrument of Fig. 2 is operative, the presence of a large air gap in the magnetic circuit is not desirable. Such an air gap introduces a large magnetic reluctance in the magnetic circuit, decreases the efliciency of the magnetic circuit and tends to produce demagnetization of the permanent magnet means included in the magnetic structure.

To eliminate the air gap represented by the passage 2? from the magnetic circuit established by the magnetic structure illustrated in Fig. 2, the connections of the magnetic circuit may be modified. The presence of two completely independent pole pieces I9 and 2| between the outer pole pieces 2 and 3 permits a substantial variety in the connections of the magnetic circuit. For example, the air gaps 5 and i may be connected in series, in parallel, or in independent magnetic circuits as desired. Suitable connections for the pole pieces are illustrated in Figs. 3 to 8.

Referring to Fig. 3, it will be observed that the outer pole pieces 2 and 3 again are connected by the magnetic conductor 9. In addition, the inner pole pieces 33 and 35, which correspond to the inner pole pieces l5 and 2| of Fig. 2, are connected by means of a magnetic conductor 31. Since the inner pole pieces 33 and 35 are magnetically connected, the passage 35 therebetween may be made substantially larger than the passage 21 of Fig. 2 without increasing the magnetic reluctance of the resulting magnetic circuit. By inspection of Fig. 3, it will be observed that the air gaps 5 and l are connected in series, the direction of fiux in the associated magnetic circuit being illustrated by the arrows For example, magnetic flux flows from the inner pole piece 33, which may be termed a north poie N, through the air gap 5, the outer pole piece 2,,

which may be termed a south pole s, the magnetic conductor 5, the outer pole piece 3, which may be termed a north pole N, the inner pole piece 35 which may be termed a south pole S and the conductor 31 to the inner pole piece 33. Any portion of the magnetic circuit may be formed of permanent magnet means capable of producing flux flow in the indicated directions. It will be observed that the directions of flux flow in the air gaps 5 and I are similar to the directions of magnetic flux flow in the air gaps of Fig. 1. Therefore, the torques which are applied to the coil ll of Fig. 3 when current flows therethrough are similar to the torques applied to the coil ll of Fig. i when the latter is energized. Since the magnetic conductors 3 and 31 are so disposed that they are external to the moving coil I when the moving coil is in the position indicated by broken lines in Fig. 3 and since they are displaced from the path of the moving coil as it r0- tates in a counterclockwise direction from the position illustrated in full lines in Fig. 3 to the position illustrated in dotted lines therein, the magnetic conductors do not interfere with the removal of the coil from the associated magnetic structure or the reinsertion of the magnetic coil in its magnetic structure. Such removal or reinsertion may be accomplished in the manner discussed with reference to Fig. 2.

Fig. 4 also discloses a circuit wherein the air gaps 5 and I are connected in series in a magnetic circuit, but the connections differ slightly from those illustrated in Fig. 3. In Fig. 4, the inner pole piece 33 is connected by a magnetic conductor 39 to the outer pole piece 3 whereas the inner pole piece 35 is connected by a magnetic conductor 4| to the outer pole piece 2. With the directions of flux flow indicated by the arrows magnetic flux flows from the inner pole piece 33 across the air gap 5 through the outer pole piece 2, the magnetic conductor 4 I, the inner pole piece 35, the air gap 1, the outer pole piece 3 and the magnetic conductor 39 to the inner pole piec 33. Any portion of this magnetic circuit may be formed of permanent-magnet material capable of producing a flow of magnetic flux in the directions indicated by the arrows.

It will be observed that the direction of magnetic flux flow in the air gap 5 is similar to that in the air gap 5 of Fig. 1. However, the direction of flow of magnetic flux in the air gap I of Fig. 4 is opposite to that of the magnetic flux in the air gap l of Fig. 1. Consequently, if the coil (1| of Fig. l were employed with the magnetic structure of Fig. 4, the torques applied to the coil would be in opposite directions about the shaft 25. For this reason, the magnetic structure of Fig. 4 is provided with a pair of moving coils 43 and 45 each of which is associated with a separate one of the air gaps 5 and 1. For example, the moving coil 43 has a side 41 disposed for movement through the air gap 5 whereas the moving coil as has a side 49 disposed for movement through the air gap i. By suitable connections of the coils, the torques applied thereto when the coils are energized may be cumulative or differential, as desired. For example, the coils may be connected in series to apply torques cumulatively to the shaft 25. On the other hand, the coils may be energized from separate sources of electrical energy to apply their torques either cumulatively or differentially to the shaft 25. This connection of the coils will be discussed at greater length below. Since the magnetic conductors 33 and M are positioned to permit movement of the asoasaa 7 moving cells 43 and 45, in a counterclockwise direction from the position illustrated in full. lines to the position illustrated in dotted lines in. Fig. 4 and since they do not pass through either of the moving coils when the moving coils are in the positions indicated by dotted lines, the moving coil assembly may be removed from its associated magnetic structure and reinserted therein in the manner discussed with reference to Fig. 2.

With reference to the modification illustrated in I'ig. 5, the air gaps and I are connected in parallel in a magnetic circuit. For example, the inner pole piece 33 and the outer pole piece 3 are connected by magnetic conductors II and 53 to one end of a magnetic conductor 55. The outer pole piece 2 and the inner pole piece 35 are connected by magnetic conductors 51 and 50 to the remaining end of the magnetic conductor 55. The directions of magnetic flux flow are indicated by arrows e. It will be observed that magnetic flux from the magnetic conductor 55 flows through the magnetic conductors 5| and 53 to the inner pole piece 33 and the outer pole piece 3 which may be termed north poles N. From these pole pieces the magnetic flux travels across the air gaps 5 and I to the outer pole piece 2 and the inner pole piece 35. Consequently, the air gaps 5 and I are connected in parallel between the ends of the magnetic conductor 55. Since the directions of flow of magnetic flux in the air saw 5 and I are similar to the directions of flow of magnetic flux in the air gaps 5 and I at Fig. 1, the coil l I may be employed in Fig. 5 and will operate in a manner similar to its operation in the instrument of Fig. 1. The magnetic conductors 5|, 55, 51, and 59 are located to permit removal of the coil I l in the manner discussed with reference to Figs. 2, 3 and 4. Any portion of the pole pieces and magnetic conductors of Fig. 5 may be formed of permanent magnet material capable of directing magnetic flux in the directions illustrated by the arrows In Fig. 6, the air gaps 5 and I are again connected in parallel between the ends of a magnetic conductor iii. For example, the inner pole piece 33 and the inner pole piece 35 are both connected to one end of the magnetic conductor I through magnetic conductors 63 and 65. The Outer magnetic pole pieces 2 and 3 are connected through magnetic conductors El and 69 to the remaining end of the magnetic conductor 5 I. Suitable directions of flow of magnetic flux are indicated by the arrows o. It will be observed that the directions of magnetic flux flow in the air gaps 5 and I are similar to the corresponding directions for the instrument of Fig. 4. Consequently, the coils 43 and 45 may be employed for the instrument of Fig. 6. The magnetic conductors ll, 83, 05, 51 and 69 are so located that. they permit removal of the coils 43 and 45 in the instrument of Fig. 6 in the same manner discussed with reference to Fig. 4. Any portion of the pole pieces or magnetic conductors may be formed of permanent magnet means capable of directing magnetic flux in the directions illustrated by the arrows o.

In Fig. '7, the air gaps 5 and I are provided with independent magnetic circuits for independent magnetic energization. The pole pieces 2 and 33 are connected by a magnetic conductor I I, whereas the pole pieces 3 and 35 are connected by a magnetic conductor I3. Suitable directions of flow of magnetic flux are indicated by the arrows e adjacent the magnetic conductors. For the speciflc directions of flow of magnetic flux indicated in Fig. '7, the directions of flow of magnetic flux in the air gaps I and 1 are similar to the directions or flow or magnetic flux 5 and I of Fig. 1. Consequently, the coil II may be employed in Fig. 7 in the same manner discussed with reference to Fig. 1. In addition, since the inner pole pieces 33 and 35 are separated, the coil I I may be removed or inserted with respect to its associated magnetic structure in its associated magnetic structure in the manner discussed with reference to Fig. 2. Any desired portion of the magnetic circuit which includes the pole pieces 2 and I3 and the magnetic conductor Il may be formed of permanent magnet material capable of directing magnetic flux in the direction indicated by the arrow 3. Similarly, any portion of the magnetic circuit which includes the pole pieces I and t5 and the magnetic conductor It may be formed of permanent magnet material capable of directing flux in the indicated direction.

B'y reversing the direction of flow oi magnetic flux in the magnetic conductor II, the direction of flow of magnetic flux in the air gap I also is reversed. Such reversal is illustrated in Fig. 8. The directions of flow of magnetic flux in the air gaps 5 and I of Fig. 8 are similar to those in the air gaps 5 and I of Fig. 4. Consequently, the coils 4i and 45 are employed in the embodiment of Fig. 8. These coils may be removed from their associated magnetic structures in the manner discussed with reierence to Fig. 4.

The pole piece constructions illustrated in Figs. 2 to 8 lend themselves admirably to the construction of a ratio-type instrument. For example, in Fig. 9 an inner pole piece 33a is provided which corresponds to the inner pole piece of Figs. 3 to 8. However, the inner pole piece 33a is configured to provide, in cooperation with the outer pole piece 2, an air gap 5a which tapers from a predetermined length adjacent its upper end (as viewed in Fig. 9) to a smaller length adjacent its lower end. In an analogous manner, an inner pole piece 35a is provided which cooperates with the pole piece 3 to provide an air gap which tapers from a predetermined length adjacent its upper end to a smaller length adjacent its lower end. The coils 43 and 45 are associated with these tapered air gaps. When the pole pieces of Fig. 9 are energized in accordance with the teachings of any of the modifications of Figs. 3 to 8, and the coils 43 and 45 are energized from independent sources of energy to apply torques diiierentially to the shaft 25, the coil assembly takes a position corresponding to the ratio between the energizations oi the two coils.

To illustrate the operation of a ratio-type instrument, let it be assumed that the coil 43 is energized from one source to produce a torque acting in the direction of the arrow I5 and that the coil 45 is energized from another source to produce a torque acting in the direction of the arrow II. Let it be assumed further that the coils 43 and 45 are similar in construction. It will be noted that the coil 48 has a side disposed in the gap So at a position wherein the gap has a 1 coil 45. Consequently, the coil assembly will 9 rotate in a clockwise direction to move the coil 43 gradually into a weaker part of the magnetic field in the air gap In as the coil 45 moves into a stronger part of the magnetic'iield in the air gap la, This rotation continues until the torques developed by the two coils are equal. This condition is assumed to exist when the coils are in the position illustrated in dotted lines in Fig. 9. If the ratio of the current in the coil 49 to that in the coil 45 increases above unity, the coil assembly will rotate still further in a clockwise direction until the torques developed by the coils again are equal. Consequently, the instrument of Fig. 9 may be calibrated to indicate the ratio between these two currents.

As above pointed out, sometimes it is expedient to use a pair of moving coils 43 and 45 in place of the single moving coil ll. Connections suitable for the moving coils 43 and 45 are 11- lustrated in Figs. 10 to 15. In Fig. 10, the two coils 43 and 45 are connected in series between two terminals 19 and through flexible conductor strips 8 la. It will be recalled that the coil 43 has a side disposed in the air gap 5, whereas the coil 45 has a side disposed in the air gap 1. The connection in Fig. 10 is assumed to be such that the currents flowing in these two sides flow in opposite directions. Such a connection of the two coils 43 and 45 is suitable when the coils are employed with the magnetic structures illustrated in Figs. 2, 3, 5 and '7. The same directions of current flow through the coils may be obtained by connecting them in parallel between the terminals 19 and 8|, as illustrated in Fig. 11. Consequently, the connections illustrated in Fig. 11 also are suitable for the magnetic structures illustrated in Figs. 2, 3, 5 and 7.

In Fig. 12, the windings 43 and 45 again are connected in series but the direction of flow of current in the coil 45 is reversed from the direction illustrated in Fig. 10. Also, in Fig. 13,

tween the terminals 19 and 8! but the directions of flow of current in the coils are similar to those obtained by the connections of Fig. 12. Consequently, the connections of Figs. 12 and 13 are suitable for the magnetic structures illustrated in Figs. 4, 6 and 8.

In Fig. 14, one lead of each of the coils 43 and 45 is connected to the terminal 19. The remaining lead of the coil 43 is connected to the terminal iii. The remaining lead of the coil 45 is connected to a terminal 93. Consequently, the coils 43 and 45 may be energized from separate sources of electrical energy and the directions of flow occurring in the two windings depend on the specific connections employed between the sources of energy and the terminals. By properly directing the flow of currents in the two windings, the connections of Fig. 14 may be employed with any of the magnetic structures employed in Figs. 2 to 9, inclusive.

In certain cases, it may be desirable to energize the two windings 43 and 45 from sources which are completely insulated from each other. In such cases, the coils '43 and 45 may have completely independent terminals 19, 85, 8| and 93, as illustrated in Fig. 15. Since the connections to these terminals may be selected to direct current through the windings-in any direction, the connections of Fig. may be employed with any of the magnetic structures illustrated in Figs. 2 to 9. For example, the directions of flow of current in the coils 43 and 45 may be selected to apply cumulative torques to the associated shaft of the instrument. The instrument then totalizes the currents supplied to the two coils when the coils are employed with the magnetic structures of Figs. 2 to 8, inclusive. Alternatively, the directions of flow of currents in the two coils may be selected to apply torques differentially to the instrument shaft. The instrument then indicates the difference between the currents energizing the two coils. Also, the two coils 43 and 45 may be employed with the magnetic structure of Fig. 9 to indicate the ratio between two currents. The terminal arrangement of Fig. 15 additionally permits the connection of the coils 43 and 45 externally of the instrument to provide the equivalent of any of the connections illustrated in Figs. 10 to 14.

To indicate more clearly suitable constructions of a permanent-magnet, moving-coil electrical instrument wherein the invention may be employed, a number of embodiments will be described in greater detail. For example, in Fig. 16 an instrument is illustrated which corresponds to that illustrated in Fig. 3. In Fig. 16, a pair of outer pole pieces 91 and 99 correspond, respectively, to the outer pole pieces 2 and 3 of Fig. 3. These pole pieces 81 and 99 are connected by a permanent magnet 9! which corresponds to the magnetic conductor 9 of Fig. 3. Between the outer pole pieces 91 and 59 are located a pair of inner pole pieces 93 and 95 which correspond to the inner pole pieces 33 and 35 of Fig. 3. Theseinner pole pieces 93 and 95 are connected by a magnetic conductor 91 which corresponds to the conductor 31 of Fig. 3. The inner and outer polepieces of Fig. 16 are spaced to define air gaps 99 and "ii which correspond to the air gaps 5 and 1 of Fig. 3. The coil Ii has two sides disposed in these air gaps 99 and Ill for movement therethrough. It will be observed that the inner pole pieces 93 and 95 of Fig. 16 are spaced to provide a passage I93 which corresponds to the passage 39 of Fig. 3. The passage I93 has a length much greater than that of the air gaps 99 and NH. By rotating the coil Ii in a clockwise direction (looking down on Fig. 16), the coil ll may be moved into alignment with the passage Hi3. When so aligned, the entire coil and its shaft 25 may be moved in an upward direction parallel to the shaft 25, or in a transverse direction toward the reader to remove the coil i l and the shaft 25 from the associated magnetic structure. Such removal does not disturb the magnetic structure in any way.

The magnetic structure of Fig. 16 may be constructed in any suitable manner. For example, the inner pole pieces 93 and 95 and the conductor 91 may be formed of unitary laminations which are secured to each other in any suitable manner, as by rivets I05. By an accurate punching operation, the surfaces of the inner pole pieces 93 and 95 which are adjacent the air gaps 99 and IM may be given an accurate shape which is assumed to be cylindrical for the embodiment here discussed.

The outer pole pieces 91 and 99 may be secured to a supporting plate I91 of non-magnetic material, such as brass, by a brazing or hard soldering operation. In addition, the permanent magnet 9| may be secured between the outer pole pieces 91 and 99 in any suitable manner as by hard soldering. After the outer pole pieces 91 and 89 are secured to the supporting plate ill'l, they may be machined to provide suitably shaped surfaces for defining the outer surfaces as of the air gaps 9 9 and "ii. These surfaces are assumed to be cylindrical for the purpose of discussion.

The assembly comprising the polepieces 91 and 99, together with the supporting plate I51 and the permanent magnet 9|, may be secured to the inner pole pieces in any suitable manner. In the specific embodiment of Fig. 16, a plate nonmagnetic material I99, such as brass, is secured to the pole piece 91 in any suitable manner as by brazing. The plate I99 then is secured to the inner pole piece assembly in any suitable manner as by means of the rivets I95.

A magnetic structure suitable for the instrument illustrated in Fig. 4 is shown in Fig. 1'1. This magnetic structure includes an outer magnetic pole piece II which is secured to one face oi a permanent magnet H1. The remaining pole face of the permanent magnet H1 is secured through a magnetic conductor II9 to an inner pole piece I2I. The magnetic circuit is completed by an outer magnetic pole piece I23 which is connected through a magnetic conductor I25 to an inner pole piece I21. The inner pole piece I21 and the outer pole piece II5 are spaced to define an air gap I29 which corresponds to the air gap 5 of Fig. 4. In a similar manner, the inner pole piece I2I and the outer pole piece I23 are spaced to define an air gap I3I which corresponds to the air gap 1 of Fig. 4. By inspection of Fig. 17, it will be noted that the air gaps I29 and I3I are connected in series across the pole faces of the permanent magnet H1.

The specific method of constructing the magnetic structure of Fig. 17 may vary appreciably. The outer pole piece II5 may be formed of solid soft steel which is machined to provide a cylindrical surface adjacent the air gap I29. This pole piece may be secured by brazing or otherwise to one pole face of the permanent magnet H1. The magnetic conductor H9 and the inner pole piece I2I may be formed as parts of unitary soft steel laminations which are secured to each other by means of rivets I33. In a somewhat similar manner, the outer pole piece I23, the magnetic conductor I25 and the inner magnetic pole piece I21 may be formed of unitary soft steel laminations which are attached to each other by means of rivets I35. One pole face of the permanent magnet I I1 is attached to the magnetic conductor I I9 in any suitable manner as by brazing. In addition, the laminations forming the magnetic conductor I I9 and the inner pole piece I2I together with the laminations forming the outer pole piece I23 and the inner pole piece I21. are suitably secured as by brazing, to a non-magnetic support I31 which may be constructed of brass. It will be observed that the inner pole pieces I2I and I21 are spaced to define a passage I39 through which the coils 43 and 45, together with the shaft 25, may be removed from operative position relative to the magnetic structure without disturbing the magnetic structure.

Fig. 18 shows a magnetic structure suitable for the embodiment of Fig. 5. In Fig. 18, an inner magnetic pole piece I and an outer magnetic pole piece I43 are connected through a magnetic conductor I45 to one pole face of a permanent magnet I41. The opposite face of the permanent magnet I41 is magnetically connected to an inner pole piece I49 and an outer pole piece I5I. The

inner pole piece I49 and the outer pole piece I43 are spaced to define an air gap I53 which corresponds to the air gap 5 of Fig. 5. The inner pole piece Ill and the outer pole piece I5I are spaced to define an air gap I55 which corresponds t pole pieces MI and I49 are spaced to define a passage I51 through which the coil II may be moved in a direction parallel to the shaft 25 for the purpose of removing the coil II from its associated magnetic structure or for the purpose of replacing the coil II in its associated magnetic structure.

Conveniently, the inner pole piece I, the outer pole piece I49 and the magnetic conductor I45 may be formed of unitary soft steel or iron laminations which may be secured to each other and to a non-magnetic plate I59 (such as one formed of brass) by means of rivets I5I. In a similar manner, the inner pole piece I49 and the outer pole piece I5I may be formed of integral magnetic laminations which are secured to each other and to the plate I 59 in any suitable manner as by means of rivets I93. The permanent magnet I41 may be inserted between the sides of the laminations and may be secured thereto and to the plate I59 in any suitable manner as by brazing. The desired pole piece configurations may be accurately formed in each of the magnetic laminations by means of a punching operation.

In Fig. 19. a magnetic structure is illustrated which is suitable for the instrument of Fig. 6. Referring in Fig. 19, a pair of inner magnetic pole pieces I55 and I51 are connected to each other by means oi a magnetic conductor I59. Conveniently, the inner pole pieces and the magnetic conductor I99 may be formed of soft iron or steel laminations which are united to each other by means of rivets "I. A pair of outer magnetic pole pieces I19 and I15 are provided. Conveniently. these pole pieces I19 and I15 may be connected by means of a link I11 to form an integral U-shaped outer pole piece assembly. The outer pole piece assembly may be formed of soft steel and may have cylindrical pole faces I19 and III machined therein. The outer pole piece I13 passes through the laminations forming the inner pole pieces and the magnetic conductor I59 and is spaced therefrom. A non-magnetic plate I99 which may be formed of brass, may be secured to the pole piece I13 in any suitable manner as by brazing and may be secured to the laminations forming the inner pole pieces and the magnetic conductor I59 in any suitable manner as by means of rivets I95. A permanent magnet I 91 has one pole face engaging the magnetic conductor I59 and one pole face engaging the outer magnetic pole piece I13. This permanent magnet may be secured in place in any suitable manner, as by a soldering operation.

The outer magnetic pole piece I19 and the inner pole piece I65 are spaced to define an air gap I99 which corresponds to the air gap 5 of Fig. 6. Similarly the outer magnetic pole piece I 15 and the inner pole piece I91 are spaced to define an air gap I9I which corresponds to the air gap 1 oi Fig. 6. These air gaps I89 and I9I are connected in parallel across the pole faces of the permanent magnet I91.

The shaft 25 may be mounted for rotation with respect to the magnetic structure by means of a bearing screw I93 which passes through a threaded opening in the link I11 and a bearing screw I95 which passes through a threaded opening in a bearing bridge I91. Since the outer pole pieces I13 and I15 are at the same magnetic potential, the bridge I91 may be formed either of non-magnetic or magnetic material, such as brass or soft steel. The bridge may be secured either permanently or detachably to the outer pole pieces as by means of machine screws I99.

The coils 43 and 45 are connected in series and the leads thereto are connected to the inner ends of a pair of flexible spiral conductor strips 2" and 293. Each of these spiral conductor strips has an inner end secured to an insulating bushing which is positioned on the shaft 25. The outer end of the spiral conductor strip 203 is connected to a terminal 255 which is secured to an insulating block 291 mounted on the outer pole piece I13. This terminal may be secured to an external electrical circuit by means of a conductor 239. In a similar manner, the outer end of the spiral conductor strip may be attached to a conductor (not shown). A control spring 2 has its inner end secured to the shaft 25 and its outer end connected to a post 2" which may be secured in any suitable manner to the magnetic structure. Preferably the outer end of the control spring is adjusted with respect to the magnetic structure for the purpose of calibration. A suitable structure for such adjustment will be discussed below. An arm H5 is secured to the shaft 25. This arm may constitute a pointer for indicating the rotation of the shaft or may constitute a pen arm if the instrument is to be employed as a recording instrument.

To remove the moving coil assembly from the magnetic structure of Fig. 19, the outer ends of the conductor strips 2lll and 203 are detached from their associated terminals. In addition, the outer end of the control spring 2 is detached from the post H3. The coils 43 and are next rotated in a clockwise direction (looking down on Fig. 19) until the coils are aligned with the pas sage 2l1 between the inner pole pieces I85 and I81. The bearing screws I93 and I95 are backed away from the shaft 25 and the complete coil assembly including the coils 43 and 45, the shaft 25, the conductor strips 2M and 203, the control spring 2 and the pen arm 2l5 are moved in a direction transverse to the shaft 25. During this movement, the coils 43 and 45 pass through the passage 2" until they are completely clear of the associated magnetic structure. Alternatively, the bridge I91, if detachable, may be removed from the outer pole pieces and the coil assembly may be removed upwardly in a direction parallel to the shaft 25 from its associated magnetic structure. An opposite procedure may be fol lowed for replacing the coil assembly in the associated magnetic structure.

Referring to Figs. 20 and 21, a, magnetic struc= ture is illustrated which is suitable for the embodiments of Figs. '1 and 8. In this magnetic structure, a pair of inner magnetic pole pieces H9 and 22! are spaced to define a passage 223 through which the coil ll may be moved. An outer magnetic polepiece 225 is spaced from the inner pole piece 2 l 9 to define an air gap 221 which corresponds to the air gap 5 of Fig. '7. In a similar manner, an outer magnetic pole piece 229 is spaced from the inner pole piece 22| to define an air gap 23l which corresponds to the air gap 1 of Fig. 7.

A permanent magnet 233 has one pole face engaging the outer pole piece 22l for directing magnetic flux into the air gap 221. In addition, a permanent magnet 235 has a pole face engaging the outer magnetic pole piece 229 for directing magnetic flux into the air gap 231.

As previously pointed out in the discussion of Figs. 7 and 8, the magnetic circuits for the two air magnets and the pole pieces to provide an eilective magnetic shield therefor. A pair of cantilever arms or horns project interiorly from opposite sides of the magnetic body 231 to form the inner pole pieces 2" and 2. It will be noted further that the permanent magnets 233 and 235 have pole faces enga i opposite interior surfaces of the magnetic body. The polarities of the permanent magnets 233 and 235 are indicated by the conventional reference characters N (north pole) and S (south pole). With the polarities indicated in Fig. 21, the magnetic structure is suitable for the instrument of Fig. 7, and the coil II is associated therewith.

The portion a-b of the magnetic body 231 which extends directly between the permanent magnet 233 and the inner pole piece 2i9 complates a magnetic circuit for flux supplied to the air gap 221 by the permanent magnet 233. This magnetic circuit is illustrated by a dotted line in Fig. 21. In a similar manner, the portion of the magnetic body 231 between the points 0 .and d (Fig. 20) completes a magnetic circuit for magnetic flux supplied to the air gap 23l by the permanent magnet 235. The portion of the magnetic body 231 between the points b and c and the portion of the magnetic body between the points a and d are employed primarily for the purpose of structurally maintaining the two magnetic circuits accurately in position with respect to each other and of completing a magnetic shield for the parts of the instrument located within the magnetic body. These latter portions may have recesses 239 and 2 formed therein to increase the effective dimensions of the passage 223.

By inspection of the drawing, it will be observed that each inner pole piece 2| 9 or 22! has a cross section which decreases from a large value adjacent its point of connection to the magnetic body 231 to a smaller value at its free end. Since the total magnetic flux in each inner pole piece also decreases from a relatively large value adjacent the point of connection of the inner pole piece to the magnetic body to a smaller value adjacent the free end of the inner pole piece, the tapered construction of the pole piece provides a reasonably low flux density throughout the pole piece while providing a passage between the inner pole piece 223 of optimum size.

Conveniently the magnetic body 231, together with the inner pole pieces, may be formed of a plurality of laminations of soft magnetic iron. Each of the laminations conforms to the contour of the magnetic body and has horns or cantilever arms projecting therefrom similar in outline to the outline of the inner pole pieces 2l9 and 228. These laminations are stacked and attached to each other by means of rivets 243 to form the magnetic body 231, together with the inner pole pieces 2l9 and 22L The contour of the inner pole pieces may be accurately controlled during the punching operation.

As shown more clearly in Fig. 22, the outer pole pieces 225 and 229, together with the permanent magnets 233 and 235, are secured to a non 1s magnetic plate such as a brass plate 245 in any suitable manner as by brazing. After the pole pieces 225 and 223 are secured to the brass plate 245, the pole pieces are machined to provide accurately formed surfaces 225a and 229a. The resulting pole piece assembly then may be inserted in the magnetic body 231 and secured thereto by means of the rivets 243. This con struction is desirable for the reason that after the machining operation, the relationship of the outer pole pieces to each other and the relationship of the inner pole pieces to each other are not disturbed. Consequently, if the outer pole pieces are provided with cylindrical surfaces 225a and 229:; and the inner pole pieces have cooperating cylindrical surfaces, the scale distribution of the resulting instrument is not critically dependent upon concentricity of the cylindrical surfaces on the inner and outer pole pieces. For example, if during assembly of the plate 245 on the magnetic body 231, the outer pole pieces 225 is moved toward the inner pole piece 2I9, it follows that the outer pole piece 225 must move away from the inner pole piece 22l. Consequently, any increase in flux density in the air gap 221 is accompanied by a corresponding decrease in flux density in the air gap 23! and the performance of the instrument is not substantially changed by such eccentric location of the outer pole pieces with respect to the inner pole pieces. To permit the utilization of small air gaps, however, it is desirable that the plate 245 be positioned accurately with respect to the magnetic body 231 during assembly of the magnetic structure. The same advantages are present in the embodiments of Figs. 16 and 19.

The shaft is positioned for rotation in a bracket assembly which will be described with particular reference to Figs. 20, 22 and 23. The bracket includes an upper bearing bridge 244 in which a bearing screw 245 is positioned to engage the upper pivot of the shaft 25. A lower bearing bridge 243 carries a bearing screw 241 for engaging the lower pivot of the shaft 25. The upper bridge has secured thereto a pair of spaced supporting posts 243 which are secured to the upper bearing bridge by means of machine screws 25!. The lower bearing bridge has secured thereto a pair of spacer rods 253 which are secured to the bridge by means of machine screws 255. As shown in Figs. 22 and 23, the posts 243 and the rods 253 both are secured to a common ring 251121 any convenient manner. For example, the posts 249 have studs projecting from their lower ends which are received in threaded openings provided in the ring 251. The rods 253 are secured to the ring by means of machine screws 253. Conveniently, the ring, bridges, posts a d rods may be formed of brass.

If desired, the bridge 243 may have an internally threaded bushing 241a secured thereto for receiving the bearing screw 241. This bushing has a cylindrical outer surface, which assists in centering the rotor assembly relative to the magnetic structui For this purpose, a bracket 24") is secured to the magnetic structure, as by brazing the bracket to the pole pieces 225 and 223. When the surfaces 225a and 229a are machined, a cylindrical opening 241c also is machined in the bracket secured to the pole faces for slidably, but snugly, receiving the bushing 241a.

By inspection of Fig. 22, it will be observed that the ring 251 has a cylindrical rim 2" projecting therefrom for reception in a cylindrical opening 263 provided in the plate 245. Since the plate 245 and ring 251 have engaging surfaces of revolution, such surfaces may be machined accurately to locate the bearing bracket correctly with respect to the pole pieces of the instrument. The ring 251 is detachably secured to the plate 245 by means of machine screws 285 which pass through the ring 251 for reception in threaded openings 251 provided in the plate 245. The plate 245 has auxiliary openings 259 communicating with the cylindrical opening 253 to permit passage of the rods 253 therethrough. These auxiliary openings 259 are in alignment with the recesses 239 and 2 of the magnetic body 231 (Fig. 20) when the plate 245 is secured to the magnetic body.

The various parts carried by the shaft 25 may be located in various positions on the shaft. For example, in Fig. 23, the coil H is wound on a rectangular former 21| which may be formed of electroconductive material to provide damping in the resultant instrument. For example, the former may be made of copper. The former and the coil carried thereby are secured to the shaft 25 by means of collars 213 which are secured to the shaft and to the former. In Fig. 23, the conductor strips are positioned below the coil H. These conductor strips are mounted on an insulating bushing having an insulating flange 215 positioned between the two conductor strips. The bushing and conductor strips may be assembled as a unit for mounting on the shaft 25. A separate lead of the coil II is connected to each of the conductor strips. The outer ends of the conductor strips 2M and 203 are connected respectively to terminals 211 and 219 in any suitable manner, as by soldering. Each terminal is fastened to an insulator 281 which is secured to one of the rods 253.

The post 2l3 has a slit 233 for receiving the outer end of the control spring 2| I. The spring end is c amped in the slit by means of machine screws 285 and may be adjusted with respect to the post 2l3 upon release of the machine screws 285. The post 2 I3 is secured to the upper bearing bridge 244. The arm 2| 5 for actuating a pen or pointer may be positioned between the control spring 2 and the bearing bridge or plate 244.

All parts of the structure in Fig. 23 which are located below the ring 251 are proportioned to pass through the opening in the magnetic body 231 formed by the passage 223 and the recesses 239 and 2 (Fig. 20) This permits the bearing bracket and coil assembly illustrated in Fig. 23 to be inserted in operative position in the magnetic structure of Figs. 20 and 21 or to be removed therefrom without disturbing the magnetic structure in any way.

In the embodiment of Fig. 20, the various parts carried by the shaft 25 are positioned in a somewhat different order from that illustrated in Fig. 23. In Fig. 20, the coil H and the conductor strips 20! and 203 are inverted to position the conductor strips above the coil. In addition, the control spring 2| l is positioned between the arm 2 l5 and the bearing bridge 24!. The outer end of the control spring 2 is secured to a lever 231 which is mounted for rotation about the axis of the shaft 25. The lever may be secured in any position of adjustment by means of the machine screw 23! which may be actuated to clamp the lever to the bearing bridge 244. A slot 29| in the lever permits adjustment of the lever with respect to the machine screw 289. Otherwise the bearing bracket assembly and the coil assembly of Fig. 20 may be similar to that illustrated in 17 Fig. 23. In Fig. 20, conductors 209 are illustrated for connecting the outer ends of the conductor strips to an external circuit.

It is believed that the operations for removing and reinserting a bearing bracket and coil assembly relative to the associated magnetic structure is clear from the foregoing discussion. I! the instrument of Figs. 20, 21 and 22 is in completely assembled condition and it is desired to remove the coil H from its associated magnetic structure, the conductors 209 (Fig. 20) are dis-: connected from the conductor strips. Next the machine screws 265 (Fig. 22) are removed to release the ring 251 from the plate 245. Finally, the coil ii is rotated in a counterclockwise direction (looking down on Fig. 20) to align the coil with the passage 223 and the entire bracket assembly, together with the coil assembly supported thereby, are raised in a direction parallel to the shaft 25 to remove the bracket assembly and coil assembly from the associated magnetic structure. During such removal, the coil ii, the rods 253 and the lower bearing bridge 248 pass freely through the opening formed by the passage 223 and the recesses 239 and 24L A reverse procedure may be followed for the purpose of reinserting the coil and bracket assemblies in the associated magnetic structure. It should be noted that the magnetic structure is not disturbed in any way during such removal and reinsertion of the bracket and coil assemblies. The opening formed by the passage 223 and the recesses 239 and 24I communicate with the air gaps 221 and 23f to provide a path through which the coil may be removed from an operative position with re: spect to its associated magnetic structure to a position external to the magnetic structure and through which the coil may be reinserted to its operative position.

To remove the coil from the magnetic structure of Figs. 20 and 21, the coil is moved in a direction parallel to the shaft 25. In some cases, it may be desirable to remove the coil in a direction transverse to the shaft 25. In such a case, the construction illustrated in Fig. 24 may be employed. In Fig. 24, a magnetic body 231a is disclosed which corresponds to the magnetic body 231 of Figs. 20 and 21, and which may be employed in place of the body 231. The only difference between these magnetic bodies resides in the provision of an opening 293 which is provided in the magnetic body 231a between the positions a and d. This opening is proportioned to permit removal of the coil assembly 294 (which corresponds to the coil ii) and its associated bearing bracket assembly from the magnetic structure in a direction transverse to the shaft 25, as represented by the arrow 295. It will be understood that the opening 293 extends not only through the laminations of the magnetic body 231a but also through the plate 245 (Fig. 20) which is secured to the magnetic laminations. The instrument of Fig. 24 otherwise may be similar in construction to that shown in Figs. 20 and 21.

The provision of the opening 293 does not interfere with the supply of magnetic fiux to the air gaps 221 and 23L The paths of flow of magnetic flux for the two air gaps are illustrated in dotted lines in Fig. 24.

To remove the coil and bracket assemblies from the associated magnetic structure in the direction of the arrow 295 of Fig. 24, the bearing bracket ring 251 must be released from the plate 245 by removal of the screws 265 (Fig. 22) in accordance with the discussion relating to Figs.

20 and 21. The bearing bracket assembly then must be raised for a distance suflicicnt to permit the rim 261 of the ring 251 to clear the plate 245. The bearing bracket assembly and the coil An opposite procedure may be followed to reinsert the bearing bracket and coil assemblies in the associated magnetic structure. It should be understood that prior to removal of the bearing bracket and coil assemblies, the coil assembly 294 must be moved to the position indicated in broken lines in Fig. 24 to align the coil assembly with the passage 223.

From inspection of Figs. 21 and 24, it may be observed that the air gaps 221 and 23i together with the passage 223 form a continuous passage having the shape of a letter S or of an inverted letter 8 depending on the end of the instrument from which the passage is viewed.

A further point is worthy of consideration.

Let it be assumed that the modification of Fig.

24 is arranged in accordance with the principles of Fig. 8. In such a case, the polarities of the permanent magnets 233 and 235 are selected to provide the pole piece polarities indicated in Fig.

The coil assembly 294 of Fig. 24 then represents the two coils 43 and 45 of Fig. 8 which are assumed to be connected in series in order to develop, when energized. torques acting in the same direction about the axis of the coil assembly. By inspection of Fig. 8, it will be tends to increase the magnetic flux in the remaining air gap, and the resultant torque acting on the coil assembly remains substantially unchanged. For this reason, the instrument is substantially immune to externally-produced magnetic field influence.

Certain structure herein shown also is incorporated in the copending patent application of L. J. Lunas, Serial No. 570,028, filed December 27, 1944, and assigned to the same assignee.

Although the invention has been discussed with reference to certain specific embodiments thereof. numerous modifications are possible. Therefore, the invention is to be restricted only by the appended claims as interpreted in view of the prior art.

We claim as our invention:

1. In a permanent-magn t. moving-coil instrument. coil means. a ma netic structure including two inner pole pieces and two outer ole pieces, said pole pieces being arran ed in pairs wherein each pair comprises an inner soft magnetic pole piece and an outer soft ma n ic pole piece spaced to define an air gap for a side of said coil means, permanent magn et-means for directing magnetic flux through said air gaps, and means mounting said coil means for rotation relative to said magnetic structure about an axis. said inner pole pieces being spaced to define a passage through which said coil means may be moved from a first position externalto said magnetic structure to a second position wherein said coil means may be rotated through a predetermined path to enter said sides of said coil means in the air gaps, said magnetic structure including integral with the 5 inner pole pieces magnetic means which, when 1% said coil means is in said second position, extends between said inner pole pieces by a path which is external to the coil means, the passage and the predetermined path.

2, In a permanent-magnet, moving-coil instrument. coil means, a magnetic structure, means mounting said coil means for rotation relative to said magnetic structure about an axis, said coil means including a pair of coil sides substantially equidistant from any point on the axis and spaced angular-1y about the axis, said magnetic structure including first and second soft magnetic inner pole pieces disposed between the coil sides. first magnetic means spaced from said first magnetic inner pole piece to define therewith a first air gap within which a first one of the coil sides is disposed for rotation, second magnetic means spaced from said second mag= netic inner pole piece to define therewith a second air gap within which a second one of the coil sides is disposed for rotation. the coil means being rotatable from a first position wherein said coil sides are disposed within their respective air gaps to an intermediate position wherein said coil sides are external to the air gaps, said inner pole pieces being spaced to define a passage permitting movement therethrough of said coil means from the intermediate position to a position external to saidmagnetic structure, and magnetic means integral with the inner pole pieces and extending between said inner pole pieces by a path external to said air gaps and said passage, said ma netic structure including permanent magnet-means for producing magnetic fields in said air gaps.

3. In a permanent-magnet. moving-coil electrical instrument, a magnetic structure, a pair of coils each having a coil side. a shaft supporting said coils for rotation about the axis of the shaft, and means mounting said coils and shaft for rotation relative to said magnetic structure, said two sides being-spaced substantially from the axis of rotation thereof, said magnetic structure comprising a unitary soft magnetic body having cantilever portions each extending through a separate one of the coils; said portions extending in opposite directions on opposite sides of said shaft to define with the remainder of said magnetic structure a separate air gap for each of said two sides of said coils. and permanent magnet-means associated with said magnetic structure for directing magnetic flux through said air gaps.

4. In a magnetic structure for a permanentmagnet, moving-coil electrical instrument, first magnetic means defining a first path for magnetic fiux, said first magnetic means comprising in series a first permanent magnet, an arcuate air gap defined by first and second pole faces and a magnetic core part, second magnetic means defining a second path for magnetic fiux independently of the first path, said second magnetic means comprising in series a second permanent magnet, an arcuate air gap defined by third and fourth pole faces and a magnetic core part, and means mounting said magnetic means in spaced relationship with said air gaps and the space between said magnetic means defining a ,continuous S-shaped passage, the first permanent magnet having a pole magnetically connected to only said first one of said pole faces, and the second permanent magnet having a pole magnetically connected to only the third one of said pole faces.

5. In a permanent-magnet, moving-coil electrical instrument, a magnetic structure including a substantially cylindrical magnetic core, a soft magnetic body substantially surrounding said magnetic core, said magnetic body having a pair of pole pieces spaced from said magnetic core and spaced angularly about the axis of said magnetic core to form therewith a pair of substantially annular air gaps, said magnetic core having a passage extending completely therethrough intermediate said air gaps to divide said magnetic core into two portions each adjacent a separate one of said pole pieces, said magnetic structure including separate magnetic means displaced from said air gaps for connecting each of said magnetic portions to said magnetic body to define a pair of magnetic paths each including a separate one of said pole pieces and a separate one of said magnetic portions, coil means proportioned for movement from a position external to said magnetic structure through said passage to a position embracing said magnetic core with coil sides located in said air gaps, and means mounting said coil means for rotation relative to said magnetic structure, said magnetic structure including separate permanent magnet-means independently effective for directing magnetic flux through each of said magnetic paths.

6. A magnetic structure for a permanentmagnet, moving-coil electrical instrument, said magnetic structure including a substantially cylindrical magnetic core, a soft magnetic body substantially surrounding said magnetic core, a pair of pole pieces spaced from said magnetic core to form therewith a pair of substantially annular air gaps, and permanent magnet-means interposed between said pole pieces and said magnetic body, said magnetic core having a passage extending completely therethrough to divide said magnetic core into two portions angularly spaced about the axis of said magnetic core, said magnetic body being magnetically connected to each of said magnetic portions at a separate point said points being spaced from each other 7. In a permanent-magnet, moving-coil electrical instrument, a coil, a unitary soft magnetic structure comprising a soft magnetic body, and a) pair of cantilever soft magnetic cores projecting from said magnetic body in opposite directions through said coil, said magnetic cores being spaced to provide a passage permitting movement of said coil therethrough from a position external to said magnetic structure to a position embracing said magnetic cores, said magnetic body extending betw een said magnetic cores by a path clear of said passage, means mounting the coil for rotation relative to the magnetic structure about an axis intermediate the magnetic cores, and permanent-magnet means associated with said magnetic cores and clear of said passage for providing magnetic fields for a plurality of sides of said coil.

8. In an electrical instrument, a unit comprising a rotor assembly, said rotor assembly including coil means, and a supporting device, said supporting device including means mounting said rotor assembly for rotation relative to said supporting device about an axis intermediate two sides of said coil means, said two sides being substantially equidistant from points on the axis, and a magnetic structure having magnetic means extending through said coil means for providing a magnetic field for each of said sides of said coil means, said magnetic structure having a. passage proportioned to permit movement of said unit from an operative position relative to said magnetic structure wherein said coil means embraces said magnetic means to a position external to said magnetic structure. V

9. In an electrical instrument, a unit comprising a rotor assembly, said rotor assembly including a coil, and a supporting device, said supporting device including means mounting said rotor assembly for rotation relative to said supporting device about an axis intermediate two sides of said coil. and a magnetic structure for providing a magnetic field for each of said sides of said coil, said magnetic structure comprising a separate magnetic path for each of said sides of said coil, each of said magnetic paths including a magnetic core extending through said coil and a magnetic pole piece positioned exteriorly of said coil to define with the associated magnetic core an air gap within which the associated side of said coil is positioned for movement, and said magnetic structure having a passage permitting movement of the unit relative to said magnetic structure from an operative position wherein said coil embraces said magnetic cores to a position external to said magnetic structure.

10. In a permanent-magnet, moving-coil instrument, coil means having a pair of sides positioned equidistant from all points on an axis and spaced angular-1y about the axis, magnetic means for establishing a separate air gap for each of said coil sides, means mounting said coil means for rotation relative to said magnetic means with said sides in said air gaps, said magnetic means including permanent magnet-means for directing magnetic fiux in opposite directions through said air gaps, said coil means including connections through which current may be directed through the coil sides in proper directions to render the instrument substantially immune to external field influence.

11. In a permanent-magnet, moving-coil instrument, a pair of coils, each of said coils having a first side adjacent a common axis and a second side displaced from the axis, independent magnetic means establishing a magnetic field for each of said second sides, each of said magnetic means comprising a permanent magnet, and means mounting the coils for rotation about said axis, said permanent magnets being poled to di rect magnetic flux in opposite directions through said magnetic fields, the coils having terminals through which current may be directed through the coil sides in proper directions to render the instrument substantially immune to external field influence.

12. The method of making a magnetic structure for a permanent-magnet, moving-coil instrument which comprises securing a pair of spaced magnetic members to a supporting member, machining said magnetic members after said securing to form opposed, substantially concave surfaces thereon, shaping a magnetic body to surround substantially said magnetic members, forming on said magnetic body spaced cantilever projections proportioned to extend from opposite sides of said body into the space between said cylindrical surfaces, and securing said supporting member to said magnetic body after said machining, whereby each of said cantilever projections defines with an adjacent one of the cylindrical surfaces a separate air gap.

13. In a permanent-magnet, moving-coil electrical instrument, a magnetic structure, a coil unit having two substantially parallel coil sides, and bearing mechanism mounting the coil unit for rotation about an axis intermediate and substantially parallel to said coil sides, the magnetic structure comprising a first inner pole piece and a first outer pole piece spaced to define an arcu-- ate and uniform air gap for a first one of said coil sides, a second inner pole piece and a second outer pole piece spaced to d fine an arcuate and uniform air gap for a second one of said coil sides, said inner pole pieces being located on opposite sides of the axis and spaced by a distance sufficient to permit movement of the coil unit from a position external to the magnetic structure'substantially in a linear direction through the passage bctween the inner pole pieces to a position wherein the two coil sides may be rotated into their respective air gaps, and a permanent-magnet means clear of said passage for directing magnetic flux through the air gaps, said coil unit comprising a pair of coils mounted on opposite sides of said axis, each of the coils providing one of said two coil sides.

14. An instrument as defined in claim 10 wherein the faces of the magnetic means bounding the air gaps are substantially concentric about the axis.

15. An instrument as claimed in claim 8 in combination with means mounting th unit on the magnetic structure with the coil sides positioned for movement through their associated magnetic fields, said passage permitting movement of the unit from its mounted position to a position external to the magnetic structure in a predetermined direction having at least a component substantially parallel to said axis, and a control spring secured to the rotor assembly, said spring being displaced from the magnetic structure in said predetermined direction when the rotor assembly is in operative position relative to the magnetic structures.

16. An instrument as claimed in claim 8 wherein the unit includes a pair of bearing means disposed on opposite sides of the magnetic structure when the unit is in operative position relative to the magnetic structures for mounting the rotor assembl for rotation relative to the support, one of the bearing means being proportioned for movement through said passage.

17. An instrument as claimed in claim 8 in combination with means mounting the unit on the magnetic structure with the coil sides positioned for movement through their associated magnetic fields, said passage permitting movement of the unit from its mounted position to a position external to the magnetic structure in a predetermined direction having at least a component substantially parallel to said axis, and a plurality of flexible convoluted conductor stripssecured to the rotor assembly, said strips being displaced from the magnetic structure in said predetermined direction when the rotor assembly is in operative position relative to the magnetic structure, said conductor strip being connected electrically to the coil means to serve as terminals therefor.

18. An instrument as claimed in claim 17 in combination with a control spring secured to the rotor assembly and an operating arm secured to the rotor assembly, said control spring and arm being displaced from the magnetic structure in said predetermined direction when the rotor assembly is in operative position relative to the magnetic structure.

19. In an electrical instrument, a unit comprising a rotor assembly, said rotor assembly including coil means, and a supporting device, said supporting device including means mounting said rotor assembly for rotation relative to said supporting device about an axis intermediate two sides of said coil means, said two sides being sub stantially equi-distant from points on the axis, and a magnetic structure having magnetic means extending through said coil means for providing a magnetic field for each of said sides of said coil means, said magnetic structure comprising a first inner magnetic pole piece and a first outer magnetic pole piece, said pole pieces being spaced to define a first air gap for a first one 01' the coil sides, a second inner magnetic pole piece and a second outer magnetic pole piece, said second pole pieces being spaced to define a second air gap for a second one of the coil sides, said inner magnetic pole pieces being positioned on op site sides of the axis and between the coil sides, and means cooperating with the pole pieces for establishing magnetic fields in the air gaps, said inner pole pieces being spaced to define a passage communicating at each end with a separate one of the air gaps, said unit being proportioned to permit rotation of the coil means into alignment with the passage and removal of the unit through the passage without disturbing the magnetic structure.

20. An instrument as defined in claim 19 wherein the coil means comprises two coils each providing a separate one of the coil sides, each inner pole pieces.

21. An instrument as defined in claim 15 wherein the removal or the unit is eflected in a predetermined direction having at least a component parallel to said axis, and a plurality of convoluted flexible strips secured to the rotor assembly at positions displaced in said predetermined direction from the coil means, said flexible strips being electrically connected to the coil means to serve as the only terminals therefor. DOUGLASS A. YOUNG. LAWRENCE J. LUNAB.

REFERENCES CITED UNITED STATES PATENTS Number Name Date 819,071 Roche et a1 May 1, 1906 1,597,327 Obermaier Aug. 24, 1926 1,646,634 Sutherland Oct. 25, 192? 1,929,292 St. Clair Oct. 3, 1933 2,235,390 Smith Mar. 18; 1941 2,313,352 Midworth Mar, 9, 1943 2,384,316 Lamb Sept. 4, 1945 2,389,393 Thomander Nov. 20, 1945 2,414,462 Grace et ai Jan. 21, 1947 FOREIGN PA'IENTS Number Country Date 13,707 Great Britain June 8, 1911 

